Elevation in CO2 concentration broadly impacts plant physiological characteristics, which influences herbivores and biotrophic pathogens, which in turn regulate the plant defensive response. In this study, responses of tobacco plants to stress in the form of the green peach aphid, Myzus persicae (Sulzer), or cucumber mosaic virus (CMV), or both aphid and CMV combined were investigated in opentop chambers under ambient and elevated CO2 concentrations. We measured aboveground biomass and foliar chlorophyll, nitrogen, non-structural carbohydrates, soluble protein, total amino acid and nicotine content in tobacco plants and also measured aphid population dynamics, body weight, honeydew production and anti-oxidative enzyme activities in individual aphids. Plants produced more secondary metabolites for defence in both CO2 treatments when treated with aphid and CMV combined than with either alone. Aphid density significantly increased on CMVinfected tobacco plants (relative to uninfected plants) under ambient CO2 but not under elevated CO2. This suggests that plant defences against virus and aphid would be more efficient under elevated CO2. Plant defence appears to shift from plant virus to aphid under increasing CO2 levels, which highlights the potential influences of multiple biotic stressors on plants under elevated CO2.

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